The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Adiabatic compression is known as the process through which gases are reduced in volume and as a byproduct, a large amount of energy is converted into heat. Most commonly, this heat is removed by a cooling fluid through heat exchangers. Immediately or eventually, most of this heat is disposed of into the environment. This heat is generally referred to as heat of compression.
Gas expansion has the opposite effect—the gas cools as it expands and most of the heat is absorbed directly or indirectly from the surrounding environment. Most gas pipelines also suffer from cooling as the gas expands and looses pressure through a pipeline, before coming to a booster station or gate station, where gas is expanded even further to reduce it to local transmission line pressures. Compressor-booster stations reside along gas pipelines to increase pressure marginally and many times, due to their minimal temperature rise during compression, they are operated without an after-cooler, leaving most of the heat of compression in the pipeline. Expander stations typically use electrical or gas-fired heaters to increase the temperature to practical levels, for example, to avoid hydrate formation.
Most compressors are driven using internal combustion engines, and these so-called drivers tend to have low energy conversion efficiencies, in the order of 25%-50%, with the rest of the energy converted to waste heat, which is disposed of into the surrounding environment.
In short, when a gas or vapor at high pressure expands through a valve into a reservoir at lower pressure, the pressure drop is accompanied by a cooling of the gas called the Joule-Thompson effect. If the gas cools too much, it can freeze in the gas line, plugging it. Additionally, if the temperature drops too low, components in the gas can condense forming droplets in the gas flow, and impurities such as water vapor can freeze on instruments and other parts causing damage.
This problem is particularly acute with wet natural gas, which is sometimes defined as natural gas that contains more than 10% C2 hydrocarbons or more than 5% C3 hydrocarbons. Wet natural gas may also contain some water, and sometimes may be saturated with water. When wet natural gas undergoes a pressure drop and expands through a valve, such as when a high pressure tank of gas is downloaded into a pipeline or to an end-user, the resultant cooling can cause the high molecular weight components of the natural gas to condense, cause impurities such as water vapor or carbon dioxide to freeze, thus subsequently clogging the line, or cause solid chemical complexes called hydrates to form, also clogging the line.
Currently, the pipe leading from an expansion valve when natural gas is downloaded is heated to prevent condensation, freezing, and the formation of hydrates. During the course of a downloading process, the pressure drop varies, the amount of cooling changes, and hence the amount of heating needed to prevent problems changes. However, the current practice is to provide an excess amount of heat at all times during a natural gas pressure letdown procedure. This is fine at the beginning of the process when the need for heat is greatest, but is a waste of energy later in the process as more heat is being put into the expanding gas than is needed to prevent condensation, freezing, and hydrate formation. Given the rising cost of energy, this is also a waste of money.
It is against this background that various embodiments of the present invention were developed.